Tumblehome

10

Upgrade the Aquila.

After obtaining the missions for the Aquila, go to the Harbormaster and besides him is a book. The book lets you do various upgrades to the Aquila. Most of them are quite expensive, but adding an additional cannon can be purchased if you don't have a lot of money.

You only need one addition for the achievement, unless you're glitched like comment 6 & 7. The book is by every harbormaster. Really wish I would've figured out the trade system before buying these, would've made raising money a whole lot easier than looting and selling pelts directly to general stores.